This invention relates to pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA) methods for gas separation, and more particularly to a method for air separation wherein the cost for the O2 product is reduced by use of a process which employs low pressure ratios and uses adsorbents exhibiting high intrinsic diffusivity.
Significant developments of the vacuum swing adsorption (VSA), PSA and VPSA methods for gas separation have taken place over the past thirty years, with major advances occurring during the last decade. Such processes have also been named subatmospheric, superatmospheric, and transatmospheric, respectively. Unless specifically otherwise noted, PSA will be used below to mean any or all of these processes. Commercalization of these processes can be attributed to improvements in the adsorbents, process cycles and advances in adsorber design.
Highly exchanged lithium molecular sieve adsorbents, as illustrated by Chao in U.S. Pat. No. 4,859,217, are representative of advanced adsorbents for O2 production. Such advanced adsorbents are expensive and represent a significant portion of the capital cost of PSA equipment.
A dominant factor in the total energy requirement of PSA processes is the ratio of adsorption to desorption pressures. Lowering the pressure ratio is a potential method of reducing power consumption. Furthermore, a reduction in PSA cycle time has the potential to reduce the amount of adsorbent required. Unfortunately, the usual consequence of both of these strategies is a reduced product (e.g., O2) recovery. Attempts to operate at lower pressure ratios have been accompanied by substantial decreases in adsorbent productivity, e.g. Leavitt, U.S. Pat. No. 5,074,892.
Smolarek, in copending U.S. patent application Ser. No. 08/964,2293, now U.S. Pat. No. 6,010,555, overcomes some of the offsetting effects of low pressure ratio through appropriate selection and operation of vacuum and compression machinery, combined with the improved flow characteristics of radial flow adsorbers. Ackley, et al. in copending U.S. patent application Ser. No. 09/622,961, have taught the maximizing of product recovery and adsorbent productivity through the use of high intrinsic diffusivity absorbents in fast cycles.
While the invention to be described below is applicable to a wide range of gas separations, PSA air separation processes aimed at the production of high purity O2 (approximately 88% to 95.7% O2) are of particular interest. Air separation prior art discussed below reflect this O2 purity range.
Advanced adsorbents of the types mentioned above are the result of improvements in equilibrium properties. Improved N2 working capacity and N2/O2 selectivity of adsorbents have been transformed into large gains in process efficiencyxe2x80x94such benefits being obtained at the expense of higher adsorbent cost (Smolarek, et al., Gas Separation Technology, 1990). Lithium-exchanged zeolite adsorbents (LiX), in particular, have had a major impact upon the evolution of PSA air separation processes. The higher N2 capacity and higher N2/O2 selectivity resulting from highly-exchanged LiX zeolites of low SiO2/Al2O3 ratio have been more recently exploited for higher performance in air separation PSA processes. Other major improvements to these processes have been the introduction of vacuum for desorption, reduction from three-bed and four-bed processes to two bed cycles, and the use of modified cycle steps such as product pressurization, purge and/or equalization. These and other prior art advances in oxygen production have been summarized by Kumar (xe2x80x9cVacuum Swing Adsorption Process for Oxygen Productionxe2x80x94A Historical Perspective,xe2x80x9d Sep. Sci. Technology, 31: 877-893, 1996).
Improving process efficiency and reducing the cost of the light component product can be accomplished by decreasing the amount of adsorbent required per unit of product (increasing adsorbent productivity) and by increasing the product recovery. The former is generally expressed in terms of bed size factor (BSF) in lbs adsorbent/TPDO (ton per day of contained O2), while the latter is simply the fraction of light component in the feed that is captured as product. Improvement in adsorbents and reduction in cycle time are two primary methods of reducing BSF.
Considerable prior art attention has been focused upon process optimization. Reiss (Chem. Ind. XXXV, p689, 1983) emphasizes the importance of adsorbent qualities and high O2 product recovery upon energy consumption in vacuum processes (VSA) for the oxygen enrichment of air. Reiss has shown that there is a minimum in the specific vacuum pump power characteristic as the desorption pressure is increased for a fixed adsorption pressure. More specifically, power per unit of O2 produced initially decreases with decreasing pressure ratio and then increases such that there is an optimum pressure ratio for minimum specific power consumption. Concurrent to the effect of decreasing pressure ratio upon specific pump power, is the uniformly decreasing amount of O2 product or a decrease in the adsorbent productivity.
Smolarek, et al. (Gas Separation Technology, 1990) achieve the objective of lowering unit O2 product cost by reducing both capital cost and power consumption. Process operating parameters were developed around advanced adsorbent characteristics. Bed size was reduced by using shorter cycles, although the optimum cycle time was selected on the basis of the minimum cost. This minimum cost was established as a compromise between decreasing bed size and decreasing process efficiency as cycle time was shortened. It was also shown that two adsorbent beds was optimum. Lower overall power consumption resulted from the combination of increased O2 product recovery, reduced adsorbent inventory and reduced equipment size. A reduction in the optimum pressure ratio was attributed to the advanced adsorbent. The type of adsorbent (LiX), the BSF (1000 lb/TPDO), and the pressure ratio (6:1), were not originally provided in the publication.
In the prior art, process optimization takes advantage of improved equilibrium adsorbent properties of higher working N2 capacity and higher N2/O2 selectivity to achieve higher overall product recovery in processes utilizing vacuum desorption. Desorption pressure was increased (pressure ratio decreased) to reduce power consumption. Cycle time was decreased to keep bed size and adsorbent cost in check. Achieving minimum pressure ratio was not a primary objective of the optimization. Indeed, the reduction in pressure ratio was limited by the accompanying increase in bed size and the reduction in product recovery. The lowest pressure ratios corresponding to xe2x80x9coptimum performancexe2x80x9d achieved in these prior art were 5:1 or higher.
Much of the prior art attends to the incremental improvement in process efficiency through cycle step modification. A good example of such improvements is given by Baksh et al. (U.S. Pat. No. 5,518,526).
The potential benefits of low pressure ratios in achieving lower power consumption have generally been limited, due to the offsetting effects of higher BSF and lower product recovery. Although adsorbents with improved equilibrium properties allow process improvement at lower pressure ratios, reductions below a critical or limiting pressure ratio have a more severe impact upon processes incorporating advanced, high cost adsorbents. In other words, the increased adsorbent inventory accompanying lower pressure ratio has a significant impact upon the capital investment of the plant. Such critical or limiting pressure ratios were defined theoretically by Kayser and Knaebel (Chem. Eng. Sci. 41,2931, 1986; Chem. Eng. Sci. 44,1, 1989) for 5A and 13X adsorbents. The O2 recovery/pressure ratio characteristics are relatively flat at higher pressure ratios (nearly constant recovery), but show a steep decline in recovery below the critical pressure ratio. The critical pressure ratio depends upon the adsorbent type and upon process operating conditions and these limits have not been well defined in practical applications. Nevertheless, the reduced recovery trends have generally discouraged PSA O2 process operation at pressure ratios below about 4:1.
More recently, Rege and Yang (Ind. Eng. Them. Res., 36: 5358-5365, 1997) presented limits for LiX zeolite and revealed an O2 recovery/pressure ratio characteristic for LiX similar to those defined earlier by others for 13X and 5A adsorbents. The theoretical results of Rege and Yang suggest pressure ratios as low as 2:1 with little penalty in O2 recovery for vacuum swing processes. They attribute this performance to the superior equilibrium properties of the adsorbent and indicate the lowest optimum BSF for their cycle to be 18 kg/kgO2 hr (1500 lb/TPDO). Adsorbent bed pressure drop and adsorbent diffusional resistance are neglected in the theoretical model. Power consumption was not considered in the analyses.
Leavitt (U.S. Pat. No. 5,074,892) proposed low pressure ratio O2 production cycles in the range of 1.4 to 4.0 for adsorbents with advanced equilibrium adsorption properties, e.g. LiX, caustic digested NaX. Leavitt""s primary motivation was to reduce overall process costs by reducing power consumption. Leavitt noted the importance of high N2 working capacity and high N2/O2 selectivity of the adsorbent and indicated the need to achieve relatively high product recovery at the low pressure ratios in order to limit the growth in BSF. Larger amounts of purge were suggested at low pressure ratio to partially offset the lower working capacity for N2. While impressive reductions in power consumption were indicated, BSF increased substantially as pressure ratio was decreased. Leavitt did not consider the effect of adsorbent intrinsic diffusivity upon process performance.
Smolarek (in copending U.S. patent application Ser. No. 08/964,293, now U.S. Pat. No. 6,010,555 has proposed a two-bed VPSA O2 cycle using a single-stage vacuum device. The adsorption pressure is in the range of 1.3 to 1.6 atm, while the desorption pressure level is between 0.4 and 0.55 atm. The preferred pressure ratio is in the range of 2.75 to 3.0. A radial flow adsorber is also utilized to provide optimum flow distribution and minimal pressure drop. The higher desorption pressure increases the molar throughput land reduces the pressure differential across the vacuum pump, resulting in the ability to select simplified (single-stage) and less costly vacuum equipment. The lower pressure ratio results in a reduction in product recovery that, in turn, requires a higher feed input for an equivalent amount of product, i.e. compared to a higher pressure ratio cycle. Cycle time is reduced, but is limited in order to avoid introducing additional inefficiencies into the process in order to keep the BSF from increasing significantly. Smolarek claims no increase in BSF compared to the higher pressure ratio reference. The effects of adsorbent properties, high adsorbent rate characteristics in particular, upon process performance have not been addressed in the teachings of Smolarek.
Reducing cycle time is a key strategy to reducing adsorbent inventory and adsorbent cost at any pressure ratio. This is even more important for low pressure ratio cycles. While shorter cycles lead to shorter beds and higher adsorbent utilization, product recovery suffers unless adsorption rate is increased. This phenomena can be ideally characterized in terms of the size of the mass transfer zone (MTZ), i.e. the mass transfer zone becomes an increasing fraction of the adsorbent bed as the bed depth decreases. Since the adsorbent utilization with respect to the heavy gaseous component is much lower in the MTZ than in the equilibrium zone, working capacity declines as this fraction increases. When the resistance to mass transfer is dominated by pore diffusion, a decrease in adsorbent particle size leads to faster rates of adsorption and smaller mass transfer zones. Unfortunately, pressure drop across the adsorbent bed increases with decreasing particle size.
Armond et al. (UK Pat. Appl. GB 2091121A, 1982) demonstrated a short-cycle ( less than 45 s)/low pressure ratio (3.0) air separation process using 5A molecular sieve. This cycle was super-atmospheric, operating with a desorption pressure near ambient. Armond apparently achieved relatively small adsorbent inventory by using very small particles (0.5 to 1.2 mm diameter) to facilitate a short cycle time. However, pressure drop through the bed (48 kPa/m) was quite high as was the power consumption, 0.7 kWhr/sm3 O2 (20 kW/TPDO). The high power consumption was presumably the result of low product recovery.
Ackley et al. in copending U.S. patent application Ser. 09/622,961 have described improved processes utilizing advanced adsorbents with high intrinsic diffusivities relative to conventional adsorbents. Increased O2 product recovery was demonstrated by increasing the rates of adsorption/desorption to create higher N2 mass transfer coefficients at a fixed pressure ratio. This concept was then applied to achieve very short cycles and very low BSF while affecting only minimal decrease in product recovery.
Notaro, et al. in copending U.S. patent application Ser. No. 09/622,867 describe a PSA air separation process, wherein the adsorbent is selected on the basis of related combinations of intrinsic rate and equilibrium properties.
Accordingly, it is a principal object of the invention to reduce product cost, reduce power consumption and increase adsorbent productivity of high performance adsorption processes for the separation of gases.
It is a further object of the invention to provide an improved PSA process for air separation.
A gas separation process incorporating the invention combines use of an adsorbent having high intrinsic diffusivity with a low pressure ratio PSA cycle. Further enhancements to the process are derived from the use of fast cycles, shallow beds and small particlesxe2x80x94especially in a radial bed configuration. The combination of low pressure ratio, high rate adsorbents and fast cycles has been found to result in an unexpected simultaneous reduction in bed size factor (BSF) and power consumption. These benefits have been achieved while minimizing a decline in product recovery through use of the high rate adsorbent. The net result is a significant reduction in product cost.
The high adsorption rate partially offsets the decline in product recovery that accompanies reduced pressure ratio, thus enabling fast cycle operation in shallow beds which affects an unexpected overall decrease in BSF. The present invention couples the effects of mass transfer rates (and the associated particle properties), cycle time and the bed depth to significantly improve gas separation efficiency at low process pressure ratios, i.e. improvements such as an increase in adsorbent productivity (lower BSF) and a decrease in process power consumption.
Both reduced cycle time and reduced pressure ratio cause a decrease in product recovery. This occurs in the former due to the increased fraction of bed devoted to the mass transfer zone and in the latter due to the decrease in selectivity or separation efficiency of the adsorbent. The reduced separation efficiency is substantial in vacuum desorption cycles using advanced adsorbents like LiX and where pressure ratio is commonly reduced by raising the desorption pressure. The application of adsorbents of high intrinsic diffusivity significantly minimizes those undesirable effects during process performance, particularly at low pressure ratios.
While this invention has been demonstrated for the case of air separation, the general methodology applies to other gas phase separations that: (1) depend upon differences in equilibrium adsorption selectivity; and (2) in which the mass transfer resistances are dominated by diffusion in the macropores of the adsorbent particles. The methodology is especially applicable to the production of oxygen in PSA processes incorporating N2-selective adsorbents, e.g. type X zeolites or advanced adsorbents such as highly Li-exchanged type X or other monovalent cation-exchanged zeolites. The invention is particularly well suited to the use of adsorbents having high capacity and high selectivity (in combination with high intrinsic diffusivity) for the most selectively adsorbed (heavy) component of the gas mixture to be separated.
The prior art has focused upon increased O2 product recovery and has exploited lower pressure ratios for lower power consumption only to the extent that was inherently allowed by the improved equilibrium properties of advanced adsorbents. Thus, with each new improvement in adsorbent capacity and selectivity, it was found that reasonable product recovery could be achieved at modestly lower pressure ratio. However, the lower working N2 capacity and shorter cycle time dictated by lower pressure ratios results in lower adsorbent productivity (higher BSF). Prior art attempts to counter this effect by reducing cycle time even further resulted in a rapid deterioration in product recoveryxe2x80x94thereby offsetting the lower power benefits of the low pressure ratio as well as limiting the potential gain in adsorbent productivity from the shortened cycle. The use of smaller particles to inhibit loss of product recovery in faster cycles is limited, in that the adsorbent bed pressure drop increases with decreasing particle size, which in turn negatively effects power consumption.
The present invention achieves higher adsorption rate through higher intrinsic diffusivity without requiring the use of very small particles (e.g. the invention preferably uses particles having an average diameter (dp) xe2x89xa70.8 mm, more preferably xe2x89xa71 mm). However, adsorbent particle size properly selected in accordance with the pore diffusivity can be applied to further enhance the benefits of the new invention.
The invention further focuses upon lowering the product cost. This approach does not demand increased product recovery; rather it demands that the cycle time, bed depth, pressure ratio, flow rate be selected in such a manner as to achieve the lowest product cost. It has been discovered that the potential benefits of low pressure ratio can be more fully exploited by the use of adsorbents modified to have high adsorptive rate (high intrinsic diffusivity), i.e. in contrast to decreasing particle size. And surprisingly, it has been found that adsorbent productivity can be maintained or even increased as pressure ratio is decreased.